1. Field of the Invention
The present invention generally relates to flat panel displays in which internal supports maintain a gap between a view screen of the display and an electron emitting surface, and, more particularly, to internal support structures used in such flat panel displays.
2. State of the Art
Cathode ray tubes (CRTs) are widely used as display monitors for television sets, computers and other devices to visually display information. CRTs exhibit images of high quality in terms of contrast, resolution, brightness and color, and respond quickly to image changes. These desirable characteristics are achieved using a luminescent phosphor coating on the interior of the CRT screen. The advantages of CRTs in terms of image quality are overshadowed in many applications by the size, weight and power consumption of CRTs, which require considerable space behind the view screen for the cathode ray tube and associated deflection yokes. Conventional CRTs are too large and cumbersome for use in certain applications including compact portable computers, instrument panels of aircraft, etc.
Substantial research and development efforts have been made to achieve a so-called "flat panel display" which does not require as great a physical depth behind the view screen as a CRT, but which exhibits display characteristics comparable to those of a CRT. These efforts have produced flat panel displays which exhibit utility in certain applications, but which do not provide the high contrast, resolution, brightness and color attributes of a CRT. Various liquid crystal technologies have resulted in displays with the desired thin depth behind the view screen, but which suffer from lack of contrast, brightness and color (or chrominance), and which are also slower in response to changes in the displayed image (screen refresh). Active Matrix LCDs have shown improved contrast, resolution, brightness, chrominance, and screen refresh, but at a very high cost and with very substantial power requirements.
Known flat panel displays use field emission cathodes and luminescent phosphors similar to those used in CRTs, but nevertheless suffer from certain inherent limitations. These flat panels displays include the matrix-addressed flat panel displays disclosed in the following U.S. Patents, which are incorporated herein by reference: U.S. Pat. No. 3,500,102 to Crost et al., U.S. Pat. Nos. 4,857,799 and 5,015,912 to Spindt et al., and U.S. Pat. No. 5,063,327 to Brodie et al. As generally described in the foregoing patents, a cathode layer is formed of a semiconducting material on a backing structure. Electron-emitting tips are formed within the semiconducting material, and electrical connections are made to the electron-emitting tips. An anode layer is formed on the inside surface of a transparent view screen, and phosphors are coated on the anode layer. The backing structure and the view screen are spaced apart by insulating spacers and sealed together to form an envelope that is evacuated. In operation, the phosphors react to bombardment by electrons by emitting visible light.
As compared to current CRT technology, which uses voltages in the 20 kV range, the flat panels displays disclosed in the foregoing patents are limited by their physical characteristics to the use of a relatively low voltage differential (well under 5 kV and typically in the range of 200 to 1000V) between the display's cathode and anode. Because of the physical arrangement of current field-emitter-based flat panel displays, attempts to use higher voltages generally results in the occurrence of a short circuit between the cathode layer and the anode layer of the device. In particular, when relatively high voltage differentials are used to power the display, the phenomenon of secondary electron emission often produces an electron multiplication effect, causing electrical shorts to develop between the cathode (electron source) and anode (view screen) along the surface of the spacers within the vacuum envelope.
Spacers or other support structures are used to prevent distortion of the view screen or backing plate of the display due to the force of atmospheric pressure upon the planar evacuated chamber. As described in U.S. Pat. No. 5,015,912 to Spindt, et al., these spacers maintain the vacuum gap between the phosphors and the cathodes at a selected distance. Distortion of either of the two planar surfaces would result in shortening of the vacuum gap, leading to the electron avalanching phenomena described in Spindt, et al.
The support structures described in the prior art generally intersect the cathode of the display at a 90.degree. angle, creating a "triple junction" of the support structure, the cathode, and the vacuum gap. Referring to FIG. 1, in a conventional field-emission flat panel display having a view screen 1, a cathode backing structure 2 and a vacuum gap 4, a support structure 3 exhibits a 90.degree. angle at the triple junction of the support structure 3, the cathode 2 and the vacuum gap 4. The trajectory of a typical secondary electron is illustrated as a series of hops 5.
Because the support structure possesses a different dielectric constant than that of the vacuum gap, an electrostatic field is generated. The triple junction causes a distortion of the field by intensifying the field at the junction as shown in FIG. 2. The distortion in the electrostatic field causes electrons to be attracted to the surface of the support structure. When these electrons impinge upon the surface of the support structure, secondary electron emission results. Secondary electrons are emitted from the surface of the support structure into the vacuum gap and are drawn toward the anode by electrical attraction. Through the action of the described electrostatic field, the secondary electrons are caused to traverse a curved path back onto the surface of the support structure, where they impinge upon the support structure surface and cause the emission of further secondary electrons, which follow a similar, though shortened, trajectory, repeating this action along the surface of the support structure toward the anode.
Because each successive cycle of secondary electron emission results in shorter trajectories, or "hops," the incidence of secondary electron emissions grows as the hops move along the support structure surface toward the anode. Since the materials commonly used for support structures exhibit high secondary electron emission characteristics, each successive hop also generates more secondary electrons. At relatively high-voltage ranges, the secondary emission of electrons is increased, and a chain reaction can result. The electrical effect of the emission of secondary electrons and their migration to and collection at the junction of the anode with the support structure is to positively charge the support structure, shortening the effective electrical length of the support structure and causing arcs to form between the anode and cathode layers (i.e., shorting out the device at its support structure).
The generally straight, smooth surfaces of prior art support structures also encourage increased secondary electron emission at higher voltages. With a flat surface, the emitted secondary electrons escape at various angles and, by action of the electrostatic field present, return to the surface of the support structure at various angles to strike the surface and cause additional secondary emissions of electrons. As shown in FIG. 3, electrons striking the surface at oblique angles will tend to plow into the surface and eject more secondary electrons than those electrons striking the surface at or nearly at right angles to the surface.
Since the physical characteristics of prior art field-emission flat panel displays limit the displays to low voltage differentials (usually around 200 to 1000V), low-voltage phosphors, which lack the luminescent efficiency of the high-voltage phosphors used in conventional CRTs, must be used.
The use of high-voltage phosphors in a field-emission flat panel display would result in a number of advantages over the use of low-voltage phosphors in such a display. High-voltage phosphors as used in current CRT displays (usually operating at voltage differentials of 20 kV and higher) require substantially less power to generate the same amount of light energy as low-voltage phosphors. (It should be noted that the very high power requirements of CRTs are not due to the use of high-voltage phosphors. The ratio to total power of the power used by a CRT to generate the image is very small. The vast majority of the power used by a CRT is used in generating and controlling the electron flow necessary to excite the phosphors through the use of an electromagnetic deflection yoke. In addition, as a result of the scanning of the electron beam across the phosphor screen, only 10% to 20% of the power of the excitation beam actually reaches the phosphors. Approximately 80% to 90% of the power of the electron beam is wasted on the shadow mask (non-illuminating) portions of the CRT screen. In a field-emission flat panel display, since there is no beam scanning, no such waste of power occurs. Power consumption is of vital concern in field-emission flat panel displays, especially where portability of the display requires the use of battery power. The same is frequently also true for small terminal displays for use in automotive or aircraft applications where the power is self-generated and is limited in supply.) Low-voltage phosphors also exhibit inferior chrominance compared to high-voltage phosphors, resulting in a comparative loss of clarity of colors and a muddy appearance of the display. Chrominance is an important consideration in achieving market acceptance of a flat panel display, as the end user has been conditioned to desire, even require, the bright color and clarity obtainable on current CRT displays.
High-voltage phosphors also exhibit a much longer average life over low-voltage phosphors (at least six times longer life). This factor is very important to market acceptance of a flat panel display, as the end user has come to expect the longer average life of the high-voltage phosphor CRT display.
High-voltage phosphors also permit the use of a metallizing layer to contain the phosphors between the metallizing layer and the transparent display screen. A metallizing layer cannot be used with low-voltage phosphors, since electron penetration varies as the square of electron energy (measured in electron volts), and the electrons emitted by a low-voltage device are less able to penetrate the metallizing layer and reach the phosphor layer in order to excite the phosphor layer and cause light energy to be generated.
One benefit of using a metallizing layer is that the metallizing layer prevents back-scattering of light emitted by the phosphor layer. For example, aluminium has a reflectivity of 90%, reflecting 90% of back-scattered light energy through the transparent view screen and thereby increasing screen brightness by approximately 70%.
Another benefit of using a metallizing layer is that it helps contain the phosphors at the screen surface. Containment is important in preventing phosphor flaking off the screen surface and contaminating the vacuum gap. Phosphor contaminants may deposit themselves in the field-emitter apertures, which interferes with their operation and may even cause shorting and destruction of the contaminated emitter. Low-voltage phosphors require the use of more binders than high-voltage phosphors to prevent phosphor flaking. The greater binder content of low-voltage phosphors further degrades their performance.
The metallizing layer also acts as an electron drain, completing the electrical circuit which begins at the emitter without interfering with the output of light from the transparent display screen. In low-voltage phosphor devices, a semitransparent conductor is usually incorporated on the transparent display screen to complete the electrical circuit. Because the conductor is semi-transparent, it reduces the output of light from the phosphors through the screen, further degrading the brightness of the display image.
The effect of the physical limitation of the prior art to a low voltage differential is to limit the device to the use of low-voltage phosphors, resulting in an image at the display screen that exhibits inferior chrominance and reduced brightness and a display that exhibits decreased life and greater power requirements than a comparable device using a high voltage differential and high-voltage phosphors.
A difficult problem in the use of high-voltage phosphors in field-emission flat panel displays, however, is that the operating voltage differentials required by such phosphors can be between 20 to 100 times those required by low-voltage phosphors. This greatly-increased voltage requires that the vacuum gap between the cathode and anode in a high-voltage flat panel display be increased by a factor of 20 to 100 times that of a low-voltage flat panel display.
The increased vacuum gap increases the demand on support structures to maintain the vacuum gap at a constant distance and to prevent distortion of the view screen and backing panel of the display. With increased vacuum gap distances, the support structure must be made taller. Maling the support structure taller results in turn in an increase in the smooth, flat support structure surface between the anode and cathode, exacerbating the secondary electron emission effect As described above, the trajectory of each successive secondary electron emission is shorter than the preceding emission. With a longer distance to travel to the anode along the support structure surface, the instances of secondary electron emissions are multiplied geometrically.
The increased height of the support structure in a high-voltage flat panel display device therefore requires methods to control and reduce secondary electron emissions.